Final answer:
Collectors are the most abundant functional group in the deposition zone of a river ecosystem, as they excel at gathering fine organic particles that settle in these areas.
Step-by-step explanation:
In a model river ecosystem, the functional groups of highest abundance at the deposition zone would most likely be collectors. These organisms specialize in gathering fine organic particles from the sediment, which are abundant in deposition zones where the water flow is slower and allows for material to settle. Unlike shredders that break down larger organic material, or scrapers that graze on algae on surfaces, collectors are adapted to this kind of environment. Predators are generally fewer in number in any ecosystem as depicted by the pyramid of predators, which states that there are lesser predators than producers for an ecosystem to function efficiently.